Transmitter control in communication systems

ABSTRACT

Techniques are described to adaptively adjust the equalizer settings of each transmitter in a transmitter-receiver pair. The transmitter-receiver pair can be used at least with implementations that comply with 40GBASE-CR4 or 100GBASE-CR10. For implementations that comply with 40GBASE-CR4, equalizer settings of four transmitters may be independently established.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/381,282, filed Mar. 10, 2009, entitled “TRANSMITTER CONTROL INCOMMUNICATION SYSTEMS”.

FIELD

The subject matter disclosed herein relates generally to techniques totransmit signals.

RELATED ART

The Institute of Electrical and Electronics Engineers, Inc. (IEEE) hasdefined numerous networking standards. For example, 10GBASE-CX4 definessignal transmission at 10 Gbps using four lanes over copper cabling.Similarly, 10GBASE-KX4 defines signal transmission at 10 Gbps using fourlanes over backplane whereas 10GBASE-KR defines signal transmission at10 Gbps over a single lane over backplane.

The evolving IEEE 802.3ba draft 1.1 (2008) standard defines 40 Gbpsoperation over copper cable. This evolving standard is also known as40GBASE-CR4. Copper coaxial cables are characterized by frequencydependent loss and exhibit loss strongly dependent on channel length.Various vendors have proposed manners to transmit at 40 Gbps over coppercable. For example, one implementation involves use of fixed transmitequalization in compliance with small form-factor pluggable (SFP+) asdescribed in IEEE Std. 802.3ae-2002 and IEEE Std. 802.3aq-2006. In thisimplementation, the transmitter circuit is coupled to a hot pluggableoptical cable using a fixed length printed circuit board trace. Fixedtransmit equalization involves establishing a desired eye opening at atransmitter side. An eye diagram represents transitions of a signal. Themore open an eye diagram, the less likelihood the signal will bemisread. However, because copper cable length varies, this scheme leadsto varying voltage levels and eye patterns at the receiver. Differenteye openings at the receiver can lead to errors in reading receiveddata.

Another approach to reproducing received signals includes use of ananalog filter at the receiver (e.g., a continuous-time linear equalizer(CTLE)). However, this approach provides suboptimal eye openingcharacteristics at the receiver. Yet another approach involves use of ananalog-to-digital converter (A/D) and digital signal processor at thereceiver. However, this approach uses high power and is complicated toimplement.

It is desirable to develop techniques to provide acceptable receivedsignal performance over copper cable at speeds over 10 Gbps such as at40 Gbps.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of example,and not by way of limitation, in the drawings and in which likereference numerals refer to similar elements.

FIG. 1 depicts a system, in accordance with an embodiment.

FIG. 2 depicts an example of a transmitter equalizer defined by FIG.72-11 of IEEE Std 802.3ap-2007.

DETAILED DESCRIPTION

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrase “in one embodiment” or “an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in one or moreembodiments.

Various authors have expressed that use of multiple cables to increasethe rate of data transmission may incur cross talk among signals on thecables and lead to unacceptable performance. For example, slide 10 ofCole, “Nx10G Electrical I/O Issues,” IEEE 802.3 Higher Speed Study Group(November 2007) indicates that cross talk from adjacent conductors,traces, and connectors may lead to unacceptable performance. However,embodiments described herein may exhibit acceptable bit error rates whenIEEE 802.3ap (2007) is applied across multiple adjacent conductors.

FIG. 1 depicts a system, in accordance with an embodiment. System 100includes transmitters 110-A and 110-B and respective complementaryreceivers 150-A and 150-B. Transmitter 110-A and receiver 150-A are partof the same remote device whereas transmitter 110-B and receiver 150-Bare part of the same local device. A receiver may request to adjustequalizer settings of each transmitter from which the receiver receivessignals. For example, receiver 150-A may request to adjust equalizersettings of transmitter 110-B. Transmitter 110-A and receiver 150-Aoperate in a similar manner as that of transmitter 110-B and receiver150-B.

For 40 Gbps operation, the transmitter-receiver pairs can be replicatedfour times for receipt of signals on four lanes. For 100 Gbps operation,the transmitter-receiver pairs can be replicated ten times for receiptof signals on ten lanes. Signals from transmitter-receiver pair channelcan be transmitted independently so that each lane operatesindependently. The number of pairs can be increased to achieve anyhigher multiple of the basic signaling rate. Also, the basic signalingrate may be higher or lower than that specified by 10GBASE-KR.

Electrical transmit and receive specifications of transmitter-receiverpairs may be based on clause 72.7.1 of IEEE 802.3ba draft 1.1 (2008).Combining signals from multiple lanes can be accomplished at a logiclevel as defined in IEEE 802.3ba draft 1.1 (2008).

Receiver 150-B includes clock and data recovery logic 152, remotetransmit (TX) adaptation engine 154, control channel decoder 156, andtransmit control 158. Clock and data recovery logic 152 may recover aclock from a signal received from transmitter 110-A and manipulate thedata (e.g., through filtering or adaptive or fixed equalization) torecover the data which was transmitted by transmitter 110-A. Clock anddata recovery logic 152 may reproduce a data signal from a signalreceived from transmitter 110-A and generate an error signal.

Control channel decoder 156 is a decoder which deciphers the answers(e.g., UPDATED, NOT UPDATED, MIN, OR MAX) received from remotetransmitter 110-A generated in response to requests from transmitter110-B to adjust equalizer settings of remote transmitter 110-A. Table72-5, FIG. 72-6, and clauses 72.6.10.2.4, 72.6.10.2.5, 72.7.1.11 in IEEEStd 802.3ap provide examples of some answers from remote transmitter110-A. Control channel decoder 156 transfers the answers to remote TXadaptation engine 154.

Control channel decoder 156 may also decipher the requests from a remotedevice to change the settings of transmitter equalizer 114 oftransmitter 110-B (e.g., increment a tap gain). Control channel decoder156 may pass the requests to TX control block 158. TX control block 158may produce answers to requests from a remote device. TX control block158 may insert the answers into the control channel for transmission tothe remote device by transferring requests to control channel encoder112.

Control channel encoder 112 inserts requests and answers into thecontrol channel, in order to send them to the remote partner. Controlchannel decoder 156 may try to change the settings of transmit equalizer114 per requests from a remote device, if possible.

Transmit equalizer 114 may include a 3-tap transmit FIR driving theanalog front-end, as described in 802.3ap (2007). For example, anembodiment of transmit equalizer is depicted in FIG. 2.

Some implementations of transmit equalizers may have a bank of Npredefined (or arbitrary) transmit equalizers of any form (e.g.,Continuous-Time Linear Equalizers (CTLE), finite impulse responsefilter, and Digital Infinite Impulse Response (IIR) filter), and apply aprotocol which scans, while handshaking, between all the equalizersettings, and chooses the best equalizer in the remote transmitter forthe receiver.

Referring again to receiver 150-B, remote transmit (TX) adaptationengine 154 may determine how to adapt the equalizer of remotetransmitter 110-A based in part on the error signal and the receiveddata from clock and data recovery 152 and an answer from control channeldecoder 156. Remote TX adaptation engine 154 passes the adaptationrequirements to control channel encoder 112 of transmitter 110-B.Control channel encoder 112 inserts these requests into the controlchannel to send them to receiver 150-A for transfer to transmitter110-A.

In one embodiment, a receiver 150-A adjusts the equalization applied bya complementary transmitter 110-A using a control channel described withregard to the 10GBASE-KR PMD control function as defined in clause72.6.10 of IEEE Std 802.3ap-2007. Of note, 10GBASE-KR is defined in IEEEStd 802.3ap-2007 for backplane but not cable. Establishing equalizersettings may take place after auto-negotiation (described later)successfully completes in order for the receiver to know the number oftransmit-receive pairs.

In some embodiments, twin axial cables can be used as a medium betweencomplementary transmitter and receiver pair. A twin axial cable mayinclude two coaxial cables, with one coaxial cable for transmitdirection and another coaxial cable for receive direction. For 40 Gbpssignal transmission, eight coaxial cables are used, namely four for thetransmit direction and four for the receive direction. However, othertypes of cable types may be used provided the specifications of clauses85.10 and 85.11 of IEEE 802.3ba draft 1.1 (2008) are met. Cables withmetal conductors other than copper can be used (e.g., any alloy such asbut not limited to silver or platinum). Optical single and multimodecables can be used. Optical cables can be used in active cableassemblies. Active means there are electrical to optical converters inthe cable assembly at each end. An equalization setting may be appliedfor each length of cable between complementary transmitter and receiverpairs.

Various embodiments of system are capable of transmitting signals atleast at 40 Gbps in compliance with 40GBASE-CR4.

FIG. 2 depicts an example of a transmitter equalizer defined by FIG.72-11 of IEEE Std 802.3ap-2007. The feed forward equalizer (FFE)structure is described at clauses 72.7.1.10 and 72.7.1.11 of IEEE Std802.3ap-2007. In this example, UI represents a clock cycle, C(1)represents a gain applied to a bit two clock cycles ago, C(0) representsa gain of a bit one clock cycle ago, and C(−1) represents a gain of thecurrent bit. An output from the transmitter equalizer is a sum ofweighted bits. For example, receiver 150-B may control the gains C(−1),C(0), and C(1). For example, remote transmit adaptation engine 154(FIG. 1) may request to adjust the gains of the transmitter equalizer.

Standard IEEE 802.3aq defines how to perform equalization at receiver tocombat inter-symbol interference at receiver. Equalizer settings atlinear optical modules of a transmitter may be adjusted to adjust an eyeat the receiver side using techniques described herein.

Adaptive transmit equalization enables enhanced performance compared tofixed equalization. For example, margins for noise (including crosstalk)and jitter may be better for adaptive transmit equalization than thoseof fixed equalization. Adaptive transmit equalization may permit abroader range of cable lengths and supported tolerances. In addition,adaptive transmit equalization may provide a simplified receiver designas compared to a system with a fixed transmitter equalizer and any formof adaptive receiver equalizer, e.g., equalization in the digital domainafter analog to digital conversion, thereby resulting in reduced powerconsumption. In addition, adaptive transmit equalization may save powerin the transmitter by using power back-off, e.g., for short channels. Aspart of the equalization, the signal power at the receiver is implicitlyknown and the signal power at the receiver can be used to request thetransmitter to reduce its power.

Embodiments of the present invention may be provided, for example, as acomputer program product which may include one or more machine-readablemedia having stored thereon machine-executable instructions that, whenexecuted by one or more machines such as a computer, network ofcomputers, or other electronic devices, may result in the one or moremachines carrying out operations in accordance with embodiments of thepresent invention. A machine-readable medium may include, but is notlimited to, floppy diskettes, optical disks, CD-ROMs (Compact Disc-ReadOnly Memories), and magneto-optical disks, ROMs (Read Only Memories),RAMs (Random Access Memories), EPROMs (Erasable Programmable Read OnlyMemories), EEPROMs (Electrically Erasable Programmable Read OnlyMemories), magnetic or optical cards, flash memory, or other type ofmedia/machine-readable medium suitable for storing machine-executableinstructions.

The drawings and the forgoing description gave examples of the presentinvention. Although depicted as a number of disparate functional items,those skilled in the art will appreciate that one or more of suchelements may well be combined into single functional elements.Alternatively, certain elements may be split into multiple functionalelements. Elements from one embodiment may be added to anotherembodiment. For example, orders of processes described herein may bechanged and are not limited to the manner described herein. Moreover,the actions of any flow diagram need not be implemented in the ordershown; nor do all of the acts necessarily need to be performed. Also,those acts that are not dependent on other acts may be performed inparallel with the other acts. The scope of the present invention,however, is by no means limited by these specific examples. Numerousvariations, whether explicitly given in the specification or not, suchas differences in structure, dimension, and use of material, arepossible. The scope of the invention is at least as broad as given bythe following claims.

What is claimed is:
 1. A transceiver comprising: a first transmitter; a first receiver; a second transmitter; and a second receiver, wherein: the first transmitter comprises an equalizer, wherein the equalizer is in compliance with one or more of clauses 72.7.1.10 and 72.7.1.11 of IEEE Std 802.3ap-2007 and wherein electrical characteristics of the first transmitter are in compliance with clause 72.7.1 of IEEE Std 802.3ap-2007; the first receiver comprises: clock and data recovery logic to determine error properties of a signal received from a third transmitter; control channel decoder to decode control signals received from the third transmitter; remote transmit adaptation engine to determine equalizer settings of an equalizer of the third transmitter based in part on the error properties and the control signals; and a transmit control logic to request to adjust settings applied by the equalizer of the first transmitter, the settings received from the third transmitter; the second transmitter comprises an equalizer, wherein the equalizer is in compliance with one or more of clauses 72.7.1.10 and 72.7.1.11 of IEEE Std 802.3ap-2007 and wherein electrical characteristics of the second transmitter are in compliance with clause 72.7.1 of IEEE Std 802.3ap-2007; the second receiver comprises: clock and data recovery logic to determine error properties of a signal received from a fourth transmitter; control channel decoder to decode control signals received from the fourth transmitter; remote transmit adaptation engine to determine equalizer settings of an equalizer of the fourth transmitter based in part on the error properties and the control signals; and a transmit control logic to request to adjust settings applied by the equalizer of the second transmitter, the settings received from the fourth transmitter; and the remote transmit adaptation engine of the first receiver and the remote transmit adaptation engine of the second receiver are to independently determine equalizer settings of the third and fourth transmitters.
 2. The transceiver of claim 1, further comprising: logic to cause transmission of the equalizer settings of the third transmitter and logic to receive an indication of whether transmitted equalizer settings of the third transmitter were applied by the third transmitter.
 3. The transceiver of claim 1, further comprising: logic to cause transmission of the equalizer settings of the fourth transmitter and logic to receive an indication of whether transmitted equalizer settings of the fourth transmitter were applied by the fourth transmitter.
 4. The transceiver of claim 1, wherein: the control channel decoder of the first receiver is to decode received signals from the third transmitter to determine whether the determined equalizer settings of the equalizer of the third transmitter are applied and the control channel decoder of the second receiver is to decode received signals from the fourth transmitter to determine whether the determined equalizer settings of the equalizer of the fourth transmitter are applied.
 5. The transceiver of claim 1, wherein: the transmit control logic of the first receiver comprises: logic to request to adjust equalizer settings of the first transmitter using a first control channel in compliance with 10GBASE-KR PMD control function defined in clause 72.6.10 of IEEE Std 802.3ap-2007 and the transmit control logic of the second receiver comprises: logic to request to adjust equalizer settings of the second transmitter using a second control channel in compliance with 10GBASE-KR PMD control function defined in clause 72.6.10 of IEEE Std 802.3ap-2007.
 6. The transceiver of claim 1, further comprising: a lane input that is to provide communicatively coupling capability to lanes, the lanes to provide signal transmission in accordance with 10GBASE-KR of IEEE Std 802.3ap-2007.
 7. The transceiver of claim 6, wherein: when lanes are communicatively coupled to the lane input, an aggregate data rate received by the transceiver comprises a product of (i) a number of lanes communicatively coupled to the transceiver and (ii) a transmission rate specified in 10GBASE-KR of IEEE 802.3ap (2007).
 8. The transceiver of claim 1, wherein the equalizers of the first and second transmitter are selected from among: Continuous-Time Linear Equalizers (CTLE), finite impulse response filter, and Digital Infinite Impulse Response (IIR) filter.
 9. The transceiver of claim 8, further comprising: logic to select an equalizer of the first transmitter based on a communication from a third transmitter and logic to select an equalizer of the second transmitter based on a communication from a fourth transmitter.
 10. The transceiver of claim 1, wherein the remote transmit adaptation engine of the first receiver and the remote transmit adaptation engine of the second receiver are to determine equalizer settings after a determination of a number of utilized transmit-receive pairs.
 11. A method comprising: determining first equalizer settings of a first transmitter of a first signal based in part on error properties of the first signal using clock and data recovery for the first signal and also based in part on control signals from the first transmitter determined using a first control channel decoding, wherein the first equalizer used by the first transmitter is in compliance with one or more of clauses 72.7.1.10 and 72.7.1.11 of IEEE Std 802.3ap-2007; determining second equalizer settings of a second transmitter of a second signal based in part on error properties of the second signal using clock and data recovery for the second signal and also based in part on control signals from the second transmitter determined using a control channel decoding, wherein the second equalizer used by the second transmitter is in compliance with one or more of clauses 72.7.1.10 and 72.7.1.11 of IEEE Std 802.3ap-2007 and wherein determining first equalizer settings is independent from determining second equalizer settings; requesting to adjust settings applied by the first equalizer of the first transmitter; and requesting to adjust settings applied by the second equalizer of the second transmitter.
 12. The method of claim 11, further comprising: receiving an indication of whether the transmitted first equalizer settings were applied by the first transmitter and receiving an indication of whether the transmitted second equalizer settings were applied by the second transmitter.
 13. The method of claim 11, further comprising: receiving third equalizer settings; receiving fourth equalizer settings; requesting application of third equalizer settings by a third transmitter using a control channel in compliance with a 10GBASE-KR PMD control function defined in clause 72.6.10 of IEEE Std 802.3ap-2007; and requesting application of fourth equalizer settings by a fourth transmitter using a control channel in compliance with a 10GBASE-KR PMD control function defined in clause 72.6.10 of IEEE Std 802.3ap-2007.
 14. The method of claim 11, further comprising: receiving a selection of an equalizer of a third transmitter and receiving a selection of an equalizer of a fourth transmitter.
 15. The method of claim 14, wherein the equalizer of the third transmitter and the equalizer of the fourth transmitter are selected from among: Continuous-Time Linear Equalizers (CTLE), finite impulse response filter, and Digital Infinite Impulse Response (IIR) filter.
 16. The method of claim 11, further comprising: selecting an equalizer of the first transmitter; selecting an equalizer of the second transmitter; transmitting the equalizer selection of the first transmitter; and transmitting the equalizer selection of the second transmitter.
 17. The method of claim 16, wherein the equalizer of the first transmitter and the second equalizer of the second transmitter are selected from among: Continuous-Time Linear Equalizers (CTLE), finite impulse response filter, and Digital Infinite Impulse Response (IIR) filter.
 18. The method of claim 11, further comprising: communicatively coupling lanes, wherein an aggregate data rate received over the lanes comprises a product of (i) a number of lanes communicatively coupled to the transceiver and (ii) a transmission rate specified in 10GBASE-KR of IEEE 802.3ap (2007).
 19. The method of claim 11, wherein determining first equalizer settings and determining second equalizer settings occur after a number of utilized transmit-receive pairs is known. 